[Swlug] DIY Geiger counter

[Alan Gray]

Hello Rhys,
I think James's suggestions are on the right track, but I am not sure you're factoring in enough allowance for the frequencies involved.

The timings I gave suggest you are working in the range 0.3 to 50 MHz i.e.radio frequencies (RF). This is quite a specialised area within electronics.

Some of the capacitor values in the signal line feel quite high. I would have expected pico value mica types.

At those frequencies components have "unexpected" properties. For instance your common resistors also have inductance and capacitance. The values are low, but in this context they do matter. Old carbon rod types have low inductance, but modern film types commonly have a spiral cut in the film to adjust the resistance and give a tight tolerance. These have more inductance.

Much the same applies to capacitors. Electrolytic types often have a rolled internal structure and much inductance. Polystyrene block types are better than the polystyrene rolled ones, but probably not as good as ceramic disc types. Best are mica ones.

The info is in manufacturers data, but unless you deal in RF you wouldn't normally look. After all it's just a simple passive component..isn't it.

Looking at your transistors. There are A, B and C variants around. C's are the highest gain. Are any of yours so marked? I think you are ok with the ones you are using, but keep in mind that RF types, like the components above, are constructed to have less capacitance.

I think you might find it useful to have a reliable test oscillator to work with. I did consider how you might build one, but then I thought you could just use a microcontroller clock. Essentially feed it into a potentiometer and tap off as much signal as you need. You'll probably need at least 1 MegaOhm connected between clock and ground so the loading has minimal effect. Start by using the scope to determine how much signal you are starting with and take it from there. Essentially you do not require a particular frequency or waveform. You simply need to be able to progressively turn down the level enough. The issue is likely to be getting it low enough.

Regards 
Alan Gray 



On 10 March 2025 22:32:01 GMT, Rhys Sage via Swlug <swlug at mailman.lug.org.uk> wrote:
>Brief answers right now. I didn't sleep well last night, did a 12 hour day and I've just come back and will be down for the night in a couple of hours/
>
>I've only tried the BC574 circuit given albeit with tweaks as of Sunday. I probably won't get to my electronics lab until Saturday. I have a somewhat exhausting job driving extra-long special needs bus routes. I'm what they call a floater - I have a route that I normally drive with a partner (whose route it actually is) but being a floater I can be thrown on any route with any group of special needs riders at the drop of a hat. Over the last couple of weeks I've had to learn and drive from memory 3 extra routes in addition to my normal route and deal with some passengers with challenging needs. I used to have my own route but when they closed the base I operated from, I chose to operate from a base local to my house rather than the main base (which is where my old base moved to). It means I am way out in the sticks, away from any shopping centres in a town where the whole town is run by one family. The Mayor, Police Chief, town council members, big town business owners are reportedly related to each other.
>
>Sunday is usually my productive day. Saturday I'm still largely recovering from the week. 
>
>I did look at the Tip120 instead of the BC547 pair but it's a slow-switching thing. I suspect  a pair of 2N2222 might be better than the BC547 pair. The switching is a lot faster. Bonus - I have a massive pile of 2N2222. I have about 30 different kinds of transistor having bought a bulk pack of different types a few years ago.
>
>The ultimate goal - after being able to read (preferably background radiation) will be to slow the output down sufficiently that I can feed it to an ATTiny13 with its 1mhz clock speed. The radiation reading will be displayed on a TM1637 4 digit LED display and might be accompanied by sound from a piezo speaker. I want to be able to have a slow click of background radiation and a high click of high radiation. 
>
>Should I look for smaller non-electrolytic capacitors? My smallest is .1uf metal film. I believe they make lower values. I normally order my electronic components from AliExpress, eBay and Temu since there are no nearby suppliers and those further away cost a lot more and save me only a few days off ordering direct from China. Radio Shack went bust or they'd be perfect.
>
>I'll have to try to unscramble the text diagrams. That will have to wait. Right now I'm feeling very tired.
>
>For those wondering how a video comes out every day - I shoot several at a time then schedule them.
>
>Rhys Sage
>
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